Economical operation of high severity hydrocarbon cracking processes and equipment requires overcoming numerous competing operational and engineering challenges. The high temperatures and process stresses can exceed the long term viability of most conventional apparatus, including conventional refractory ceramics. In addition to component physical and thermal performance considerations, component chemical inertness and crystalline stability also are significant considerations. Pyrolysis temperatures combined with the presence of carbon from the hydrocarbon feedstock and potential presence of oxygen present special ceramic-metallurgical challenges to avoid premature ceramic corrosion.
Conventional steam crackers are a common tool for cracking volatile hydrocarbons, such as ethane, propane, naphtha, and gas oil. Other higher severity thermal or pyrolysis reactors are also useful for cracking hydrocarbons and/or executing thermal processes, including some processes conducted at temperatures higher than can suitably be performed in conventional steam crackers. Compared to conventional cracking equipment and processes, higher temperature reactions (e.g., >1500° C.) and processes typically require more complex, costly, and specialized equipment to tolerate the intense heat and physical stress conditions. Properties such as melt temperature, reaction environment non-inertness, component strength, and toughness limitations commonly define limits for many processes.
In addition to physical temperature limitations for reactor materials, many prior art ceramic reactor materials that are relatively inert at lower temperatures become susceptible to chemical degradation, ceramic corrosion, and/or crystalline alteration at higher temperatures, leading to premature degradation and/or process interference such as by generation of unacceptable levels of contaminants in the process. Although high temperature regenerative pyrolysis reactors are generally known in the art as capable of converting or cracking hydrocarbons, they have not achieved widespread commercial use, due significantly to the fact that they have not been successfully scaled to a commercially economical size or useful life span as compared to less severe alternatives, such as steam cracking.
For example, the “Wulff” process represents one of the more prominent prior art processes for generation of acetylene. Wulff discloses a cyclic, regenerative furnace, preferably including stacks of Hasche tiles (see U.S. Pat. No. 2,319,679) as the heat exchange medium. However, such materials have demonstrated insufficient strength, toughness, and/or chemical inertness, and are not amenable to use as certain desirable reactor components, such as for use as reactor fluid conduits, to facilitate large-scale commercialization. Prior art regenerative pyrolysis reactor systems have typically used alumina as the bed packing material. The commercial embodiments of these reactor systems did not operate at temperatures sufficient to achieve high conversion of methane feed. One reason for this is that pure alumina has a melting point of 2050° C. and practical alumina will have lower melting point due to the effect of impurities. Maximum practical use temperatures are typically two- to three-hundred degrees lower than the actual melting temperature, which combined with decreases due to impurities, renders alumina unsuitable for use in a high temperature (e.g., >1500° C., or >1600° C., or up to 2000° C.) pyrolysis reactor. Although some of the “Wulff” art disclose use of various refractory materials, a commercially useful process for methane cracking or other extreme high-temperature processes has not previously been achieved utilizing such materials. The aforementioned practical obstacles have impeded large scale implementation of the technologies. Materials availability for high temperature, high-stress applications is one of the most critical issues in design and operation of large-scale, commercial, high-productivity, thermal reactors.
While many ceramics tend to be somewhat inert or chemically stable at moderately elevated temperatures, many ceramics become chemically and/or structurally unstable at high temperatures, tending to volatize, change chemical phases, carburize, degrade, and/or corrode within undesirably short periods of time. Exemplary chemically and/or thermally unstable ceramics include but are not limited to certain silicas, aluminas, borides, carbides, and nitrides. Many of such ceramics are also known to undergo alterations in crystalline structure at elevated temperatures and/or across relevant process temperature ranges. Such alterations can also result in changes in volume resulting in creation of stress fractures which in turn may reduce the material's strength or thermal performance properties.
For example, zirconia is a crystalline material that is commonly used in certain refractory ceramics. However, zirconia undergoes a crystalline change between moderately high temperatures and severely high temperatures in the way its atoms are stacked (polymorphic transformation). Zirconia has a monoclinic crystal structure between room temperature and about 1200° C. Above about 1200° C., zirconia converts to a tetragonal crystal structure. At a still higher temperature, such as above 2370° C., zirconia changes from tetragonal to cubic structure and melts at 2715° C. These transformations are accompanied by volumetric shrinkage and expansion between the crystalline states, resulting in fractures or cleavages along grain boundaries. In polycrystalline zirconia, this tetragonal-monoclinic transition and cleaving results in a progressive reduction in strength and potential failure. Stabilizers, such as yttria (Y2O3) and some other metal oxides can sometimes be incorporated within the crystal structure to arrest or prevent the crystalline shifts, rending the crystal structure more stable across a broader temperature spectrum. However, such incorporation may not be sufficient to prevent undesirable thermodynamic alterations, such as from the oxide phase to a carbide phase.
It has been learned that certain stabilizers are more volatile and susceptible to progressive high temperature loss than other stabilizers. More volatile stabilizers are frequently less desirable than more loss-resistant stabilizers. For example, calcia (CaO) and magnesia (MgO) stabilizers are capable of providing a stabilized ceramic that initially achieves many of the desirable performance properties, but over time calcia and magnesia stabilizers may be more susceptible to loss than other less volatile stabilizers.
A similar, recently recognized problem particular to high temperature hydrocarbon pyrolysis pertains to carburization within the ceramic component, which can produce carbide-oxide conversion chemistry in the ceramic zirconia oxide that also leads to progressive component degradation, herein considered a type of “ceramic corrosion.” This high temperature hydrocarbon-related corrosion mechanism was not previously identified, understood, or recognized as a concern with high temperature hydrocarbon pyrolysis using ceramics. Carburization is a heat activated process in which a material, such as a ceramic or metal, is heated to temperature below its melting point, in the presence of another material that liberates carbon as it thermally decomposes, such as hydrocarbons. The liberated carbon can permeate the exposed surface and near-surface interior of the ceramic crystal matrix and either remains in spatial regions as coke or at more elevated temperatures react with the ceramic to form ceramic carbides. Such permeation by carbon can over time adversely affect the mechanical and chemical properties of the ceramic material such as are otherwise needed for long-term use in commercial, hydrocarbon pyrolysis reactors. Ceramic component volatility and progressive loss due to the severe temperatures and cyclic temperature swings also may contribute to carburization. Issues include carbon infiltration and coking within the ceramic matrix pores and an associated, undesirable carbide-oxide interaction chemistry resulting in progressive corrosion and degradation of the ceramic matrix, including micro-fractures due to coke expansion. Such problems are of particular interest in high severity pyrolysis of hydrocarbon feedstocks (e.g., >1500° C.).
The pyrolysis art needs a ceramic composition that resists or avoids carbon permeation, carburization, and/or oxide-carbide corrosion. The desired materials should concurrently provide and maintain the needed structural integrity, crystalline stability, relatively high heat transfer capability, and chemical inertness required for large scale, commercial applications, particularly with respect to hydrocarbon pyrolysis.